This invention relates to an air/fuel ratio control method for feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine in response to oxygen concentration in the exhaust gases emitted from the engine, and more particularly to a method of this kind which enables the engine to positively continue its operation even when an abnormality occurs in the oxygen concentration detecting means.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Pat. No. 3,483,851, which is adapted to determine the valve opening period of a fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concetration (oxygen concentration), etc., by electronic computing means.
According to this proposed fuel supply control system, the air/fuel ratio control is effected such that when the engine is in normal operating condition, the valve opening period of the fuel quantity metering or adjusting means is controlled in closed loop mode, whereas when the engine is in any of predetermined particular operating conditions other than the normal operating condition, such as an idling region, a mixture leaning region, a wide-open-throttle region, and a decelerating region, the valve opening period is controlled in open loop mode wherein a corresponding one of predetermined coefficients having predetermined values appropriate to respective such particular operating conditions is applied, so as to achieve a desired air/fuel ratio appropriate to such particular operating condition, thereby improving the fuel consumption and driveability of the engine.
It is thus desirable that a predetermined air/fuel ratio corresponding to each of the particular operating conditions can be achieved with certainty by means of open-loop control. However, as a matter of fact, the actual air/fuel ratio can sometimes have a value different from a desired predetermined value due to variations in the performance of various sensors for detecting the operating condition of the engine and a system for controlling or driving the fuel quantity metering or adjusting means. In such event, it is impossible to obtain required operational stability and driveability of the engine.
To overcome such disadvantage, there has been proposed by the assignee of the present application in U.S. Pat. No. 4,445,482 a fuel supply control method which is improved over the aforementioned proposed fuel supply control system, and in which a mean value of values of a first coefficient applied during feedback mode control of the air/fuel ratio effected in response to detected values of the oxygen concentration in the engine exhaust gases is calculated and stored as a second coefficient, and the second coefficient is used for control of the air/fuel ratio in open loop mode, thereby achieving air/fuel ratios closer to predetermined or required air/fuel ratios corresponding to the respective particular operating conditions.
However, even with such improved method, when an abnormality occurs in the functioning of the oxygen concentration detecting means, such as a disconnection in the wiring, a proper air/fuel ratio cannot be achieved, resulting in an abnormal air/fuel ratio of the mixture being supplied to the engine, if no countermeasure is taken to cope with such abnormality.